omgosh are you serious only 4mpg pulling 15.5K that's horrific!!!!
I pulled 10K with a 351 Windsor HO with a 4bbl and a C6 trannie in my 85 F150 - yes I know it was way too much weight but I did it all the time.
I would get 6.5 to 7.5 with it running 65 to 70 mph.
my 6.0 p/s gets 12 to 13 pulling that kind of weight if I roll through the hills vs hammering on it.
If I hammer it 9 to 10